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DNA REPLICATION
DNA replication is semiconservative
• 1 parental helix -> 2 daughter strands
• Should be identical
(may not be because of
mutations)
• Each new strand is made up of ½ of original strand
Replication Forks
• The first step of replication is the separation of the two
strands of the helix
• DNA helicase (enzyme) untwists the double helix at the
replication origin
• As it unwinds it forms 2 “Y” shapes called replication forks
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• Single-stranded binding proteins(SSB) coat each strand
and keep it apart
• RNA primase(enzyme) adds first nucleotide to the strand
at replication origin
• DNA polymerase (enzyme) adds the other nucleotides to
the 2 parent strand
• It can only add them in one direction (5’->3’)
Leading Strand
• Since this strand is orientated 5’->3’ nucleotides are
added one at a time by DNA polymerase
• This is called continuous replication
Lagging Strand
• DNA polymerase can only work 5’->3’
• RNA primase bonds to the DNA strand and adds a short
primer sequence.
• DNA polymerase then replaces the RNA nucleotides and
finishes the segment
• The segments that form are call Okazaki fragments
• DNA ligase then binds all the fragments together
• The lagging strand is called discontinuous replication,
since small fragments form
Watch the following animation
• http://highered.mcgraw-
hill.com/sites/0072943696/student_view0/chapter3/animat
ion__dna_replication__quiz_1_.html